In fixed-bed hydroprocessing reactors, gas and liquid reactants (e.g. hydrogen and a hydrocarbonaceous feedstock) flow downward through one or more beds of solid catalyst. (See, e.g. U.S. Pat. No. 4,597,854 to Penick).
As the reactants flow downward through the reactor catalyst beds, the reactants contact the catalyst materials and react to produce the desired products. Gas reactants such as hydrogen are consumed, and heat is generated by the catalytic reactions. Controlling the temperature of the feedstock as it travels downward through the reactor is important to ensure the quality and quantity of product yield is maximized toward the target product(s).
Cool hydrogen-rich gas can be introduced between the catalyst beds to quench the temperature rise and replenish the hydrogen consumed by the reactions. In order to maintain overall reactor performance, the temperature of the fluids within the reactor should be as uniform as possible and liquids and gases should be well mixed in order to maximize performance. Poor interbed fluid mixing can limit reactor operation in various ways. When interbed mixing is unable to erase the radial temperature differences, these differences persist or grow as the process fluids move down the reactor. Hot spots in any bed can lead to rapid deactivation of the catalyst in that region which shortens the total reactor cycle length. Product selectivities are typically poorer at high temperatures. For example, hot regions can cause color, viscosity and other product qualities to be off-specification. Also, if the temperature at any point exceeds a certain value (typically 800 to 850° F.), the exothermic reactions may become self-accelerating leading to a runaway event, which can damage the catalyst, the vessel, or downstream equipment.
Due to these hazards, refiners operating with poor reactor internal hardware must sacrifice yield and/or throughput to avoid the deleterious effects of poor interbed fluid mixing. Reactor temperature maldistribution and hot spots can be minimized through mixing and equilibration of reactants between catalyst beds, correcting any temperature and flow maldistributions, and minimizing pressure drops. The mixing of fluids between catalyst beds can be accomplished through the use of distributer assemblies and mixing chambers. With present-day refinery economics dictating that hydroprocessing units operate at feed rates far exceeding design, optimum interbed fluid mixing is a valuable low-cost debottleneck.
Distributor assemblies can be used to collect, mix, and distribute fluids in the interbed region of multi-bed catalyst reactors. Distributor assemblies generally include a trough for collecting and mixing liquid and gas flowing from an overhead catalyst bed, and a mixing device or chamber disposed centrally within the trough for receiving liquid from the trough and further mixing the liquid and gas.
The mixing device is a key component of many distributor assemblies because it provides efficient and thorough mixing of fluids/gases and helps avoid hot spots and poor temperature distribution.
The mixing device has at least one inlet for receiving liquid from the trough and at least one outlet for directing flow toward an underlying catalyst bed. Designs for mixing devices vary, including baffle mixer designs such as ribbon blenders and disk-and-donut type mixers that promote mixing through changing the direction of the fluid and gases.
Another type of mixer is a centrifugal or vortex-type design. This type of mixer collects the liquid and gas streams flowing downward through the reactor, and introduces them into a circular chamber where they make several rotations before being passed downward through a centrally located aperture.
If present, the mixing device is generally located in the interbed space between catalyst beds in a reactor. The interbed space in many reactors is limited due to the presence of support beams, piping, and other obstructions which occupy the interbed region. Due to these space constraints, unique hardware, such as a mixing device scaled to fit the space available, is required to perform efficient two-phase mixing in what amounts to limited volume. In addition, lower height distributor assemblies can increase catalyst loading volume with the same reactor volume, therefore improve utilization of the reactor volume.
Due to the importance of sufficient interbed fluid mixing for good catalyst lifetimes, high throughput, long cycle length, and overall reactor performance, improved mixing devices are needed. In addition, mixing devices that have lower vertical footprint and that can be retrofitted to existing reactors which have limited interbed space are of particular necessity.